1. Field of the Invention
This invention relates to a switching power supply circuit and, more specifically, to a switching power supply circuit useful when detecting a current flowing in the circuit, and providing feedback on current information to obtain a predetermined output voltage.
2. Description of the Related Art
In a switching power supply circuit, a current feedback mode is in wide use for maintaining the stability of the switching power supply circuit.
This type of switching power supply circuit concerned with an earlier technology is shown in FIG. 17. As shown in this drawing, the illustrated switching power supply circuit is a step-up switching power supply circuit, which comprises a switching means SW1 formed from an N-channel MOS transistor, a switching means SW2 formed from a P-channel MOS transistor, and an inductor L, for converting a direct current input voltage VIN into a predetermined direct current output voltage VOUT.
An error amplifier 1 constituting a feedback control system outputs an error signal Verr which represents a differential voltage between a preset reference voltage VREF and a voltage obtained by dividing the direct current output voltage VOUT by resistances R01 and R02. A PWM comparator 2 constituting the feedback control system compares the error signal Verr with an oscillation signal RAMP outputted by an oscillation circuit 3, and exercises on/off control of the switching means SW1 to SW2 by a switching signal SWS of a predetermined duty determined based on the comparison. More concretely, a buffer circuit 4 constituting the feedback control system forms switching signals SWS1 and SWS2, based on the switching signal SWS, such that the switching means SW2 is OFF during the ON-period of the switching means SW1, thereby turning the switching means SW1 and the switching means SW2 on and off alternately. In the drawing, C01 signifies an output capacitor, and C02 signifies a speed-up capacitor.
Such a switching power supply circuit further provides the feedback control system with a current feedback system for providing feedback on current information in order to stabilize control. That is, based on a current ISW flowing to the switching means SW1, a current information signal S11 is formed by utilization of a voltage at the point of connection between the switching means SW1 and a current detection resistance Rsen. The current information signal S11 is supplied to a differential amplifier 6 to obtain a current superposed signal S12, which is added to the aforementioned error signal Verr (substantially, subtracted from the error signal Verr, because the sign of the current superposed signal S12 is minus) at an adder 5, whereby a new error signal Verr-r is obtained. This error signal Verr-r is compared with the oscillation signal RAMP by the PWM comparator 2.
The current information signal S11 is supplied to an inverting input terminal of the differential amplifier 6 having a non-inverting input terminal grounded.
With the above-mentioned earlier technology, the current ISW flowing into the switching means SW1 is formed into the current information signal S11 by utilization of the voltage at the point of connection between the switching means SW1 and the current detection resistance Rsen, and the current information signal S11 is supplied to the differential amplifier 6, and further supplied to the adder 5 of the feedback control system. That is, the current ISW of the switching signal SWS1 is amplified by the differential amplifier 6, which is also an amplifier. This poses the problem that a processing speed is low and a response is poor. As a result, this earlier technology has been difficult to apply to a high frequency switching power supply circuit, in particular.
To improve the conversion efficiency of this type of switching power supply circuit, it is necessary to lower the resistance values of the switching means SW1 and the current detection resistance Rsen connected to the switching means SW1. If the resistance values become low, however, a voltage drop caused at the current detection resistance Rsen by the current ISW is decreased. As a result, the influence of the offset voltage of the differential amplifier 6 increases, presenting the problem of rendering the action of the switching power supply circuit unstable.
Like the earlier technology shown in FIG. 17, known patent documents, namely, Japanese Patent No. 3636321 (patent document 1) and Japanese Unexamined Patent Publication No. 1998-225105 (patent document 2), can be named as disclosing switching power supply circuits having current feedback systems. However, these technologies pose problems similar to those of the earlier technology shown in FIG. 17 that has been described above. That is, they have the problem of a low processing speed and a poor response. They encounter the problem, in particular, that their application to a high frequency switching power supply circuit is difficult, and they essentially render the action of the switching power supply circuit unstable.